Method of measuring the alkyl chloride concentration in an electrolyte

ABSTRACT

THE ALKYL CHLORIDE CONCENTRATION IN THE ELECTROLYTE OF AN ELECTROLYTIC CELL IS MEASURED AND/OR RECORDED BY DETERMINING THE CONCENTRATION OF THE CHLORIDE IN THE GAS PHASE OF THE ELECTROLYTE. THE GAS PHASE IS AMALYZED BY AN INFRARED ANALYZER WHICH IS SENSITIVE TO THE CHLORIDE, AND CALI-   BRATED TO READ OUT THE WEIGHT PERCENT OF CHLORIDE IN THE LIQUID PHASE OF THE ELECTROLYTE.

March 2,1971 o, WALKER 3,567,606

METHOD OF MEASURING THE ALKYL CHLORIDE CONCENTRATION IN AN ELECTROLYTE Filed Aug. 8, .1969

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ANALYSER CELL CELL I cELL INVENTOR ALFRED O. WALKER ATTORNEYS United States Patent Oifice 3,567,600 Patented Mar. 2, 1971 Int. Cl. B01k 3/00 US. Cl. 204-59 9 Claims ABSTRACT OF THE DISCLOSURE The alkyl chloride concentration in the electrolyte of an electrolytic cell is measured and/or recorded by determining the concentration of the chloride in the gas phase of the electrolyte. The gas phase is analyzed by an infrared analyzer which is sensitive to the chloride, and calibrated to read out the weight percent of chloride in the liquid phase of the electrolyte.

This invention relates in general to monitoring the alkyl chloride content of the process liquid in an electrolytic cell during electrolysis by infrared analysis, and more particularly to determining the alkyl chloride concentration in the electrolyte of an electrolytic cell which produces organo metallic compounds such as tetraalkyl lead compounds by electrolyzing a solution of a Grignard reagent in the presence of a sacrificial lead anode.

The production of organo metallic compounds by electrolyzing a liquid electrolyte in the presence of a sacrificial anode requires proper control of concentrations in the electrolyte. More particularly, manufacture of tetraalkyl lead compounds, including, for example, tetramethyl lead and tetraethyl lead, requires the electrolyzing of a solution of Grignard reagent in the presence of a sacrificial lead anode in a suitable electrolytic cell. Such a cell is disclosed in US. Patents 3,287,249 and 3,368,961. An example of some of the electrolytes used is disclosed in US. Patent 3,380,900. Alkyl chloride is used continually in the electrolyzing of the electrolyte, and it is therefore continually added to the electrolyte. It is necessary to maintain close control over the alkyl chloride concentration in such an electrolyte in order to produce efficient cell operation and to manufacture an economical product. Specifically, the methyl chloride in a cell producing tetramethyl lead should be between 0.8 to 2 percent by weight, while the ethyl chloride in a cell producing tetraethyl lead should be 6 to 10 percent by weight. In either type of cell, should the concentration of the alkyl chloride fall below the minimum, there is a danger of magnesium needles or crystals forming on the cathode and shorting out the cell. This danger is especially present at the end of a run when the active Grignard concentration is low. It is quite apparent that shorting out of the cell will cause shutdown in order to recondition same and downtime for a cell is costly to the production of the lead compounds. Should the chloride concentration rise above the level of the maximum point in the range, loss of active Grignard and alkyl chloride due to the Wurtz-Fittig reaction will cause formation of ethane and the wastage of magnesium metal thereby materially reducing the economical operation of the cell.

Heretofore, the concentration of alkyl chloride in the electrolyte has been determined by gas chromatography analysis of the liquid phase. Analysis of the liquid phase requires considerable time. The gas phase chromatography analysis method is based on Henrys law that the partial pressure of a soluble gas in the gas space above a liquid is proportional to the concentration in the liquid. Accordingly, a gas sample, taken under controlled temperature and pressure conditions, is analyzed by gas chromatography for determining the alkyl chloride concentration. A factor, previously determined, is then used to convert this gas concentration to weight percent alkyl chloride in the liquid phase of the cell electrolyte. According to the measurement, control of the alkyl chloride feed into the electrolyte permits control of the concentration in the electrolyte. Due to the time required for sampling and analysis, it is not practical to make an analysis on a single cell oftener than every four hours.

The analysis method of the present invention comprises sampling of the gas phase to pass it through an infrared analyzing instrument. Alkyl chlorides have a strong absorption peak in the infrared spectra at 700 to 750 cm.- Other gases and vapors present, such as nitrogen, methane, ethane, tetrahydrofuran, and toluene do not absorb in this region of the spectra at the concentration present in the gas. Thus, the chlorides are measured here by infrared absorption.

The production of organo metallic compounds is accomplished by a system which includes an electrolytic cell of the type disclosed in the aforementioned cell patents and a surge drum or tank connected to the cell to provide a reservoir for excess electrolyte in the recirculation of electrolyte through the cell. During a cell run, the electrolyte is continually recirculated and mixed by pumping it through the cell and the surge drum. The alkyl chloride concentration in the electrolyte is continually depleated during electrolyzing of the electrolyte, and is replenished by continual feeding of alkyl chloride into the electrolyte in the recirculating path. As above mentioned, the level of chloride concentration must be maintained within a predetermined range in order to obtain economical production of the organo metallic compound, and this requires analysis of the electrolyte to determine the level of chloride concentration. A large gas space is maintained in the surge drum above the liquid phase of the electrolyte to thereby also define in the drum a gas phase of the electrolyte. A gas sample is drawn from the gas space of the surge drum, and a small part of that sample is passed through an infrared analyzer sensitive to alkyl chloride vapor of the gas phase. The analyzer is calibrated to read out the percent by weight concentration of alkyl chloride in the liquid phase of the electrolyte in view of the proportional relationship of the chloride concentration in the gas and that in the liquid. With this information, the operator can control the feed of alkyl chloride into the electrolyte in order to maintain a predetermined concentration of chloride which enhances the economical production of the organo metallic compound of the cell. When the analyzer is connected to a single cell, a substantially continuous concentration reading can be made, or if connected to a plurality of cells, a reading can be made about every minute for a cell. In the latter case, if there are twelve cells in a total system, each cell could be analyzed about every twelve minutes.

It is therefore an object of the present invention to provide a new and improved method for determinin the alkyl chloride concentration in an electrolyte used in the production of organo metallic compounds.

Another object of this invention is in the provision of a method of measuring the alkyl chloride concentration in an electrolyte, wherein the measurement can be quickly and accurately made thereby reducing the time necessary for such analysis and ultimately providing closer control over the operation of an electrolytic cell thereby producing more efficient and economical operation and a more economical product. Still another object of this invention trolyte for determining alkyl chloride concentration to avoid undesirable high and low chloride concentration.

A still further object of this invention is to provide a method of controlling the amount of alkyl chloride concentration in an electrolyte during operation of an electrolytic cell by measuring and/ or recording the alkyl chloride concentration in the gas phase of the electrolyte.

Another object of this invention is in the provision of a method for quickly and accurately determining the alkyl chloride concentration in an electrolyte during operation of the electrolytic cell, thereby enabling prompt correction of the concentration level, and resulting in the saving of raw material, especially magnesium metal, and also providing more economical production of organo metallic compounds. A still further object of this invention is in the provision of an instrumental method of alkyl chloride analysis thereby removing the possibility of an operator erring by sampling the wrong cell.

Other objects, features and advantages of the invention will be apparent from the following detailed disclosure, taken in conjunction with the accompanying sheet of drawing, wherein like reference numerals refer to like parts, in which:

FIG. 1 is a diagrammatic view of a single cell system and illustrating the use of the infrared analysis method according to the invention; and

FIG. 2 is a view similar to FIG. 1 but illustrating a plural cell system.

In general, the method of analyzing an electrolyte to determine the alkyl chloride level according to the invention may be applied to a system including a single cell or a system including a plurality of cells. Continual measurement of the chloride level is possible with a single cell, while in a plural cell arrangement periodic measurement of the chloride level in each cell may be taken at intervals depending upon the total number of cells. The operator of the cell system, upon reading the alkyl chloride level, can increase, decrease, or maintain the feed of the alkyl chloride to the electrolyte. Alternately, an automated arrangement may be provided for automatically controlling the alkyl chloride feed to the electrolyte in response to the infrared analyzer.

A cell system includes an electrolytic cell of the types disclosed in the above-mentioned cell patents which in general include a cathode arranged with a sacrificial lead anode in spaced relation so that an electrolyte, such as a solution of a Grignard reagent, can be passed therebetween and electrolyzed to produce a tetraalkyl lead compound. The electrolyte is continually recirculated through the cell during a cell run. The length of time for electrolysis is directly dependent on batch size. An alkyl chloride is continually fed or injected into the electrolyte to enhance the electrolytic reaction, and it is important that the level of alkyl chloride concentration be maintained within a predetermined range in order to produce proper and eflicient cell operation. The electrolyte is recirculated through a surge drum having a large gas space to define therein a liquid phase and a gas phase. Recovery of the tetraalkyl lead compound may be accomplished in any suitable manner such as that disclosed in the aforementioned U.S. Patent 3,380,900.

A sample of the gas phase of the electrolyte is drawn from the gas space of the surge drum, and a small part of this sample is passed through an infrared analyzer to determine the level of alkyl chloride. The infrared analyzer is sensitive to alkyl chloride vapors to indicate the relative amount in the gas. Calibration of the infrared analyzer permits a direct readout of the level of alkyl chloride concentration in the liquid phase of the electrolyte. This permits, the operator to adjust, if necessary, the alkyl chloride injected into the electrolyte to maintain the chloride concentration level within the desired range for proper operation of the cell system. The cell is operated at a fixed temperature of about 140 and the electrolyte is maintained under a fixed pressure so that the gas phase pressure is about 10 pounds per square inch in the surge drum.

The infrared analyzer employed is a non-dispersive type which uses a double beam and a single sensor. The flow sample of gas to be analyzed is introduced into one beam path and a comparison sample is introduced into the other beam path, wherein the sensor responds to changes in the alkyl chloride level. The analyzer is especially sensitized to reduce interferences from other vapors present and to respond more accurately to the alkyl chloride level. The instrument responds to the amount of alkyl chloride in the infrared spectra at 700 to 750 CHIC-1, where the chlorides have a strong absorption peak. Exemplary of the type of analyzer that may be used is the M-S-A Lira Analyzer made by Mine Safety Appliances Company.

For the production of tetramethyl lead compounds, methyl chloride is injected into the electrolyte. It is desirable to maintain the methyl chloride at about 1 percent by weight level during the cell run, and in particular within the range of 0.8 to 2 percent by weight. Should the chloride level drop below the desired operating range, there is a danger of magnesium needles or crystals forming on the cathode and thereby shorting out the cell, especially near the end of the run when the active Grignard concentration is low. Shorting out of the cell necessitates shutdown and reconditioning which is costly to the production of the lead compound. Should the chloride level rise above the range, formation of ethane by the Wurtz-Fittig effect and the wastage of magnesium metal is caused. Accordingly, in order to maintain economical operation of the tetramethyl lead cell it is important that the level of methyl chloride be within the above range.

Ethyl chloride is injected into the electrolyte in the production of tetraethyl lead compounds, and in order to maintain economical operation of this type of cell, the level of ethyl chloride in the electrolyte must be maintained at 6 to 10 percent by weight. Again, operation of the chloride level above or below this range results in possible shorting out of the cell or the loss of Grignard and chloride due to the Wurtz-Fittig reaction.

It should be recognized that the method of analyzing the alkyl chloride level in an electrolyte according to the invention may also be applied to other types of electrolytic cells.

Referring to the drawings to further illustrate the method according to the present invention, and first to FIG. 1, an electrolytic cell 10 is shown in connected relation with a surge drum 11. The electrolyte is recirculated in the cell system by pumping the electrolyte preferably through the cell from the upper end downwardly to the lower end, although the direction of the electrolyte path may be otherwise. A recirculating line 12 connects the outlet of the cell 10 to the inlet of the surge drum 11, while a recirculating line 13 connects the outlet of the surge drum 11 to the inlet of the cell 10. Within the surge drum 11, a large gas space 14 is provided above the electrolyte, thereby effectively defining a liquid phase 15 and a gas phase 16 within the drum. A gas sample on the order of 30 cubic feet per hour is withdrawn from the gas space 14 through the line 17. A small portion of the gas sample is delivered through line 18 to an infrared analyzer 19 to pass through and permit measuring of the alkyl chloride which may be read out by the indicator 20. The small amount of gas passed through the analyzer, on the order of about 200 cubic centimeters per minute, may be dumped to waste through line 21. That portion of the gas sample drawn from the gas space 14 and not passed through the analyzer is sent through line 22 to a dry pad system for solvent recovery of vapors. Alkyl chloride is continually fed into the electrolyte through line 23, and the amount may be adjusted by the operator in accordance with the readings of the infrared analyzer. It should be recognized that the analyzer will be calibrated to read out the percent by weight of the chloride concentration in the liquid electrolyte. A continual reading of the alkyl chloride concentration may be taken by the analyzer in this single cell system. It should also be appreciated that the output of the infrared analyzer may be applied directly to a control valve for automatically adjusting the feed of alkyl chloride into the electrolyte in accordance with the desired range of concentration.

An electrolytic cell system producing tetraalkyl lead compounds which includes a plurality of cells, and which utilizes the alkyl chloride analyzing method according to the invention is illustrated in FIG. 2. For simplicity, only two cells are shown in the system, although any number may be included. Cell 24 has its outlet connected to a surge drum 25 by a recirculating line 26, and the inlet connected to the surge drum by the recirculating line 27. A large gas space 28 is provided in the surge drum by maintaining the level of the liquid phase of the electrolyte at a predetermined point to thereby define a liquid phase 29 and a gas phase 30. Similarly, a second cell 31 has its outlet connected to the inlet of a surge drum 32 by recirculating line 33, and a recirculating line 34 connects the outlet of the surge drum to the inlet of the cell for enabling recirculation of the electrolyte. The level of the electrolyte in the surge drum is such that a large gas space 35 is provided thereby defining in the drum a liquid phase 36 and a gas phase 37.

An infrared analyzer 38 of the same type as the analyzer 19 in FIG. 1 is provided with a selector switch 39 for analyzing the gas phase of cell 24 or the gas phase of cell 31. A sample of gas may be removed from the gas space in the surge drum 25 by the line 41, and a portion of this sample may be passed through the analyzer by line 42. The remaining part of the sample is recovered and injected back into a dry pad system through recovery line 44. Similarly, a sample of the gas phase may be withdrawn from the drum 36 by line 45, and a small portion of this sample may be passed through the analyzer by line 46, while the major portion is recovered and injected back into the dry pad system through recovery line 47. That gas which is analyzed by the infrared analyzer 38 may be dumped to waste by the line 48.

Alkyl chloride is continually injected into the electrolyte ahead of cell 24 by the line 49, and similarly, alkyl chloride is continuously injected into the electrolyte of the cell 31 by the line 50.

In operation, the operator may control the selector switch 39 to either test for the alkyl chloride concentration in the electrolyte of cell 24, or to test for alkyl chloride concentration in the electrolyte for the cell 31 or this switching can be done automatically. A direct reading is taken from the analyzer indicator 51. Accordingly, this cell system differs from that of FIG. 1 only in that a plurality of cells may be arranged so that a single infrared analyzer can measure and/or record the alkyl chloride concentration in the electrolyte of any cell in the system. The operator can thereby increase, decrease or maintain the alkyl chloride feeding to the electrolyte of any cell in view of the alkyl chloride concentration. Alternately, a signal from the analyzer could automatically control alkyl chloride feed.

Suitable valves and pumps, not shown, are provided to control liquid and gas flow in the systems of FIGS. 1 and 2.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

This invention is hereby claimed as follows:

1. The method of measuring the alkyl chloride concentration in the electrolyte of an electrolytic cell producing organo metallic compounds by electrolyzing a solution of a Grignard reagent in the presence of -a sacrifical metallic anode, and maintaining a given concentration of alkyl chloride in said electrolyte, said method comprising the steps of (a) recirculating the electrolyte in the cell and through a surge drum;

(b) maintaining the level of electrolyte in the surge drum to define a gas space therein;

(0) continually feeding a given amount of alkyl chloride into the electrolyte;

(d) maintaining the temperature and pressure of the electrolyte constant to maintain a constant gas pressure in said surge drum gas space;

(e) drawing a sample of the gas from the gas space;

(f) passing at least a part of the gas sample through an infrared analyzer sensitive to the alkyl chloride vapor in the gas to determine the alkyl chloride concentration therein;

(g) reading out the concentration in terms of the weight percent alkyl chloride in the electrolyte; and

(h) controlling the feed of alkyl chloride into the electrolyte to maintain the given concentration therein.

2. The method of maintaining a predetermined concentration of alkyl chloride in the electrolyte of an electrolytic cell producing tetraalkyl lead compounds by electrolysis of a solution of a Grignard reagent in the presence of a sacrificial lead anode which comprises (a) continually recirculating the electrolyte through the cell and a surge drum wherein the electrolyte level in the surge drum is such as to provide a gas space therein and thereby define liquid and gas phases;

(b) continually injecting a predetermined amount of alkyl chloride into the electrolyte;

(c) maintaining the temperature and pressure of the electrolyte constant to maintain a constant gas pressure in the surge drum gas space;

(d) drawing a sample of the gas phase from the gas space;

(e) passing at least part of the gas phase sample through an infrared analyzer sensitive to the alkyl chloride vapor in the gas phase to determine the concentration of same;

(f) reading out the alkyl chloride concentration in terms of weight percent of alkyl chloride in the liquid phase; and

(g) controlling the amount of alkyl chloride to be injected into the electrolyte to maintain the desired predetermined concentration therein.

3. The method as defined in claim 2, wherein the cell produces tetramethyl lead compounds and the alkyl chloride used is methyl chloride, and wherein the predetermined concentration of methyl chloride is about 0.8 to 2 weight percent.

4. The method as defined in claim 2, wherein the cell produces tetraethyl lead compounds and the alkyl chloride used is ethyl chloride, and wherein the predetermined concentration of ethyl chloride is about 6.0 to 10 weight percent.

5. The method as defined in claim 2, wherein the sample in step (d) is about 30 cubic feet per hour.

6. The method as defined in claim 5, wherein the part in step (e) is about 200 cubic centimeters per minute.

7. The method as defined in claim 2, wherein the temperature in step (c) is about F.

8. The method as defined in claim 7, wherein the pressure in the gas space in step (c) is about ten pounds per square inch.

9. The method of maintaining a predetermined concentration of alkyl chloride in electrolytes of a system of electrolytic cells producing tetraalkyl lead compounds by electrolyzing a solution of a Grignard reagent in the pres ence of a sacrificial le-ad anode, which comprises (a) continually recirculating the electrolyte of each cell through the cell and a corresponding surge drum wherein the electrolyte level in each drum is such as to provide a large gas space therein and thereby define liquid and gas phases of the electrolyte;

(b) continually injecting a predetermined amount of alkyl chloride into each electrolyte;

(c) maintaining the temperature and pressure of each electrolyte constant to maintain a constant gas pressure in the corresponding surge drum gas spaces;

(d) selectively and periodically connecting each gas space to an infrared analyzer sensitive to the alkyl chloride vapor in the gas phase;

(e) drawing a sample of the gas phase from the connected gas space and passing at least a part thereof through said analyzer to measure the alkyl chloride concentration;

(f) reading out the alkyl chloride concentration in terms of weight percent of alkyl chloride in the liquid phase; and

(g) controlling the amount of alkyl chloride to be injected into the electrolyte analyzed to maintain the desired concentration therein.

References Cited TA-HSUNG TUNG, Primary Examiner N. A. KAPLAN, Assistant Examiner US. Cl. X.R. 

